Yet the
pistil of each cabbage-flower is surrounded not only by its own six
stamens, but by those of the many other flowers on the same plant.
How, then, comes it that such a vast number of the seedlings are
mongrelized? I suspect that it must arise from the pollen of a
distinct VARIETY having a prepotent effect over a flower's own pollen;
and that this is part of the general law of good being derived from
the intercrossing of distinct individuals of the same species. When
distinct SPECIES are crossed the case is directly the reverse, for a
plant's own pollen is always prepotent over foreign pollen; but to
this subject we shall return in a future chapter.

In the case of a gigantic tree covered with innumerable flowers, it
may be objected that pollen could seldom be carried from tree to tree,
and at most only from flower to flower on the same tree, and that
flowers on the same tree can be considered as distinct individuals
only in a limited sense. I believe this objection to be valid, but
that nature has largely provided against it by giving to trees a
strong tendency to bear flowers with separated sexes. When the sexes
are separated, although the male and female flowers may be produced on
the same tree, we can see that pollen must be regularly carried from
flower to flower; and this will give a better chance of pollen being
occasionally carried from tree to tree. That trees belonging to all
Orders have their sexes more often separated than other plants, I find
to be the case in this country; and at my request Dr. Hooker tabulated
the trees of New Zealand, and Dr. Asa Gray those of the United States,
and the result was as I anticipated. On the other hand, Dr. Hooker has
recently informed me that he finds that the rule does not hold in
Australia; and I have made these few remarks on the sexes of trees
simply to call attention to the subject.

Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair.
As yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which
offers so strong a contrast with terrestrial plants, on the view of an
occasional cross being indispensable, by considering the medium in
which terrestrial animals live, and the nature of the fertilising
element; for we know of no means, analogous to the action of insects
and of the wind in the case of plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence of
two individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here currents in the water offer an obvious means
for an occasional cross. And, as in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities,
namely, Professor Huxley, to discover a single case of an
hermaphrodite animal with the organs of reproduction so perfectly
enclosed within the body, that access from without and the occasional
influence of a distinct individual can be shown to be physically
impossible. Cirripedes long appeared to me to present a case of very
great difficulty under this point of view; but I have been enabled, by
a fortunate chance, elsewhere to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, in the
case of both animals and plants, species of the same family and even
of the same genus, though agreeing closely with each other in almost
their whole organisation, yet are not rarely, some of them
hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the
difference between hermaphrodites and unisexual species, as far as
function is concerned, becomes very small.

From these several considerations and from the many special facts
which I have collected, but which I am not here able to give, I am
strongly inclined to suspect that, both in the vegetable and animal
kingdoms, an occasional intercross with a distinct individual is a law
of nature.